There have been proposed a number of amorphous silicon films for use as an element member in semiconductor devices, image input line sensors, image pickup devices or the like. Some such films have been put to practical use.
Along with those amorphous silicon films, there have been proposed various methods for their preparation using vacuum evaporation technique, heat chemical vapor deposition technique, plasma chemical vapor deposition technique, reactive sputtering technique, ion plating technique and light chemical vapor deposition technique.
Among those methods, the method using heat chemical vapor deposition technique (hereinafter referred to as "CVD method") had been tried once in various sectors, but nowadays it is not used because elevated temperatures are required and a practical deposited film can not be obtained as desired.
On the other hand, the method using plasma chemical vapor deposition technique (hereinafter referred to as "plasma CVD method") has been generally recognized as being the most preferred and is currently used to manufacture amorphous silicon films on a commercial basis.
However, for any of the known amorphous silicon films, even if it is such that is obtained by plasma CVD method, there still remain problems unsolved relating to its characteristics, particularly electric and optical characteristics, deterioration resistance upon repeating use and use-environmental characteristics. The solutions to those problems must correlate with its use as an element member for the foregoing devices and also for other points such as its homogeneity, reproductibility and mass-productivity.
Now, although the plasma CVD method is widely used nowadays as above mentioned, it is still accompanied with problems since it is practiced under elevated temperature conditions. Other problems are presented in the process, including the apparatus to be used.
Regarding the former problems, because the plasma CVD method is practiced while maintaining a substrate at elevated temperature, the kind of the substrate to be used is limited to those which do not contain a material such as a heavy metal, which can migrate and sometimes cause changes in the characteristics of the deposited film to be formed. Secondly, the substrate thickness is likely to be varied on standing in the plasma CVD method. Therefore, the resulting deposited film, lacking in uniformity of thickness and in homogeneity of composition, can exhibit changed characteristics.
Regarding the latter problems, the operation conditions to be employed under the plasma CVD method are much more complicated than the known CVD method, and it is extremely difficult to generalize them.
That is, there already exist a number of variations even in the correlated parameters concerning the temperature of a substrate, the amount and the flow rate of gases to be introduced, the degree of pressure and the high frequency power for forming a layer, the structure of an electrode, the structure of a reaction chamber, the rate of flow of exhaust gases, and the plasma generation system. Besides said parameters, there also exist other kinds of parameters. Under these circumstances, in order to obtain a desirable deposited film product, it is required to choose precise parameters from a great number of varied parameters. Sometimes, serious problems occur. Because of the precisely chosen parameters, a plasma is apt to be in an unstable state. This condition often invites problems in a deposited film to be formed.
And for the apparatus in which the process using the plasma CVD method is practiced, its structure will eventually become complicated since the parameters to be employed are precisely chosen as above stated. Whenever the scale or the kind of the apparatus to be used is modified or changed, the apparatus must be so structured as to cope with the precisely chosen parameters.
In this regard, even if a desirable deposited film should be fortuitously mass-produced, the film product becomes unavoidably costly because (1) a heavy investment is firstly necessitated to set up a particularly appropriate apparatus therefor; (2) a number of process operation parameters even for such apparatus still exist and the relevant parameters must be precisely chosen from the existing various parameters for the mass-production of such film. In accordance with such precisely chosen parameters, the process must then be carefully practiced.
Against this background, there is now an increased demand for a method that makes it possible to practice the process at lower temperatures and at a high film-forming rate in a simple apparatus in order to mass-produce a desirable functional deposited film having a relevant uniformity and having many practically applicable characteristics and such that the product will be relatively inexpensive.
Likewise, there is a similar situation which exists with respect to other kinds of functional films, such as silicon:nitrogen film, silicon:carbon film and silicon:carbon film and silicon:oxygen film.